Modulation of cerebellar excitability by polarity-specific noninvasive direct current stimulation

The cerebellum is a crucial structure involved in movement control and cognitive processing. Non-invasive stimulation of the cerebellum results in neurophysiological and behavioral changes, an effect that has been attributed to modulation of cerebello–brain connectivity. At rest, the cerebellum exerts an overall inhibitory tone over the primary motor cortex (M1), cerebello-brain inhibition (CBI), likely through dentate-thalamo-cortical connections. The level of excitability of this pathway before and after stimulation of the cerebellum, however, has not been directly investigated. In this study we used transcranial magnetic stimulation (TMS) to determine changes in M1, brainstem and CBI before and after 25 minutes of anodal, cathodal or sham transcranial direct current stimulation (tDCS) applied over the right cerebellar cortex. We hypothesized that anodal tDCS would result in an enhancement of CBI and cathodal would decrease it, relative to sham stimulation. We found that cathodal tDCS resulted in a clear decrease of CBI, whereas anodal tDCS increased it, in the absence of changes after sham stimulation. These effects were specific to the cerebello-cortical connections with no changes in other M1 or brainstem excitability measures. The cathodal effect on CBI was found to be dependent on stimulation intensity and lasted up to 30 minutes after the cessation of tDCS. These results suggest that tDCS can modulate in a focal and polarity-specific manner cerebellar excitability, likely through changes in Purkinje cell activity. Therefore, direct current stimulation of the cerebellum may have significant potential implications for patients with cerebellar dysfunction as well as to motor control studies

Disrupting the ventral premotor cortex interferes with the contribution of action observation to use-dependent plasticity

Action observation (AO), observing another individual perform an action, has been implicated in several higher cognitive processes including forming basic motor memories. Previous work has shown that physical practice (PP) results in cortical motor representational changes, referred to as use-dependent plasticity (UDP), and that AO combined with PP potentiates UDP in both healthy adults and stroke patients. In humans, AO results in activation of the ventral premotor cortex (PMv), however, whether PMv activation has a functional contribution to UDP is not known. Here, we studied the effects disruption of PMv has on UDP when subjects performed PP combined with AO (PP+AO). Subjects participated in 2 randomized-crossover sessions measuring the amount of UDP resulting from PP+AO while receiving disruptive (1Hz) transcranial magnetic stimulation (TMS) over the fMRI activated PMv or over orbito-frontal cortex (FC, Sham). We found that unlike the sham session, disruptive TMS over PMv reduced the beneficial contribution of AO to UDP. To ensure that disruption of PMv was specifically interfering with the contribution of AO and not PP, subjects completed two more control sessions where they performed only PP while receiving disruptive TMS over PMv or FC. We found that the magnitude of UDP for both control sessions was similar to PP+AO with TMS over PMv. These findings suggest that the fMRI activation found in PMv during action observation studies is functionally relevant to task performance, at least for the beneficial effects that AO exerts over motor training